Chapter 14, Machines
Physical Science, McDougal-Littell, 2008
SECTION 1 (PP. 449-457): MACHINES HELP PEOPLE DO WORK.
Georgia Standards: S8P3c – Demonstrate the effect of simple machines (lever, inclined plane,
wedge, pulley, screw, and wheel and axle) on work; S8CS3c – Apply the metric system to
scientific investigations that include metric to metric conversions (i.e.- centimeters to meters).
1. Machines change the way force is applied.
A machine is a device that helps people do work. It does not change the amount of work
done.
o If a machine decreases the amount of force needed to do the work, the distance
over which that force is applied increases.
o A machine can change the direction of the applied force.
Input force is the force applied to a machine. Output force is the force exerted by the
machine. The number of times a machine multiplies the input force is the machine’s
mechanical advantage (MA). To find a machine's mechanical advantage, divide the
output force by the input force.
2. Work transfers energy.
A machine increases the potential or kinetic energy of an object by doing work on it.
When you use a machine to do work, there is always an exchange, or tradeoff, between
the force you use and the distance over which you apply that force. You can use less
force over a greater distance or a greater force over a shorter distance to do the same
amount of work.
J. C. Booth Middle School
Physical Science (8A)
Coach Dave Edinger
Chapter 14, Machines
Physical Science, McDougal-Littell, 2008
3. Output force is always less than input force.
Efficiency is the ratio of a machine’s output work compared to its input work.
You can calculate the efficiency of a machine by dividing the machine's output work by
its input work and multiplying that number by 100.
Recall that work is measured in joules. Suppose you do 600 J of work in using a rope
system to lift a box. The work done on the box is 540 J. You would calculate the
efficiency of the rope system as follows:
No real machine is 100 percent efficient. Machines lose energy due to friction, which is
why we lubricate machine parts. Another source for loss of efficiency is air resistance,
which is the reason for streamlined designs (aerodynamics) for everything from
airplanes, automobiles, or bicycle helmets.
CALCULATING EFFICIENCY:
Example
A person is doing 1000 joules of work on a hammer to pry up a nail. The hammer does 925 joules of
work on the nail to pull it out of the wood.
J. C. Booth Middle School
Physical Science (8A)
Coach Dave Edinger
Chapter 14, Machines
Physical Science, McDougal-Littell, 2008
(1)
Find the ratio of output work to input work.
(2)
To convert the decimal to a percent, multiply 0.925 by 100 and add a percent sign.
ANSWER The efficiency of the hammer is 92.5 percent. This means that the hammer loses 7.5
percent of the input work to friction and other products.
SECTION 2 (PP. 458-467): SIX SIMPLE MACHINES HAVE MAY USES.
Georgia Standards: S8P3c – Demonstrate the effect of simple machines (lever, inclined plane,
wedge, pulley, screw, and wheel and axle) on work; S8CS3f – Use ratios and proportions,
including constant rates, in appropriate problems.
1. There are six simple machines.
The lever and the inclined plane are the two main types of simple machines. Other
simple machines are based on these two.
o A lever is a solid bar that rotates on a fixed point call a fulcrum. There are three
classes of levers based on the relative positions of the fulcrum, input force, and
output force.
J. C. Booth Middle School
Physical Science (8A)
Coach Dave Edinger
Chapter 14, Machines
Physical Science, McDougal-Littell, 2008
o A wheel and axle is a wheel attached to a shaft. It acts like a rotating collection of
levers. The input force can be applied to either part, which transfer force to the
other part.
o A pulley is a wheel and axle and a grooved rim. A rope or cable moves in the
groove. Pulleys can either be fixed or moveable. A fixed pulley allows you to take
advantage of the downward pull of your weight to move a load upward. It does
not, however, reduce the force you need to lift the load. Also, the distance you
pull the rope through is the same distance that the object is lifted. To lift a load
two meters using a fixed pulley, you must pull down two meters of rope. In a
movable pulley setup, one end of the rope is fixed, but the wheel can move. The
load is attached to the wheel. The person pulling the rope provides the output
force that lifts the load. A single movable pulley does not change the direction of
the force. Instead, it multiplies the force. Because the load is supported by two
sections of rope, you need only half the force you would use with a fixed pulley to
lift it. However, you must pull the rope through twice the distance. A
combination of two types of pulleys is called a block and tackle.
o An inclined plane is a sloping surface that supports the weight of an object while
the object moves from one level to another.
o A wedge has a thick and a thin end. A wedge can be used to cut, split, or pierce
objects or to hold objects together.
o A screw is an inclined plane wrapped around a cylinder or a cone to form a spiral.
Screws can be used to hold things together or to raise or lower objects.
2. The mechanical advantage of a machine can be calculated.
The number of times a machine multiplies the input force is the machine's mechanical
advantage. You can calculate a machine's mechanical advantage using this formula:
This formula works for all machines, regardless of whether they are simple machines or
more complicated machines.
If a machine decreases the force you use to do work, the distance over which you have to
apply that force increases. It is possible to use this idea to calculate the mechanical
advantage of a simple machine without knowing what the input and output forces are. To
make this calculation, however, you must assume that your machine is not losing any
work to friction. In other words, you must assume that your machine is 100 percent
efficient. The mechanical advantage that you calculate when making this assumption is
called the ideal mechanical advantage.
o For an inclined plane, divide the length of the incline plane by the height of the
incline.
J. C. Booth Middle School
Physical Science (8A)
Coach Dave Edinger
Chapter 14, Machines
Physical Science, McDougal-Littell, 2008
o For a wheel and axle, divide the radius of the input force by the radius of the
output force.
o For a lever, calculate this by dividing distance from the input force to the fulcrum
by the distance from the output force to the fulcrum.
o For pulleys, the mechanical advantage is equal to the number of ropes that
support the weight of the object.
SECTION 3 (PP. 468-475): MODERN TECHNOLOGY USES COMPOUND MACHINES.
Georgia Standards: S8P83c - Demonstrate the effect of simple machines (lever, inclined
plane, wedge, pulley, screw, and wheel and axle) on work; S8CS5a – Observe and explain how
parts can be related to other parts in a system such as the role of simple machines in complex
machines.
1. Compound machines are combinations of simple machines.
A machine that is made of two or more simple machines is a compound machine.
Compound machines often have many moving parts and must overcome more friction
than simple machines.
Gears are based on the wheel and axle. Gears have teeth on the edge of the wheel that
allows one gear to turn another. A set of gears forms a compound machine in which one
wheel and axle is linked to another.
Two linked gears that are the same size and have the same number of teeth will turn at
the same speed. They will move in opposite directions. In order to make them move in
the same direction, a third gear must be added between them. The gear that turns another
gear applies the input force; the gear that is turned exerts the output force. A difference in
J. C. Booth Middle School
Physical Science (8A)
Coach Dave Edinger
Chapter 14, Machines
Physical Science, McDougal-Littell, 2008
speed between two gears—caused by a difference in size and the distance each turns
through—produces a change in force.
The mechanical advantage of a compound machine equals the product of the mechanical
advantage of all of the simple machines that make up the compound machine. For
example, if a lever with a mechanical advantage (MA) of 2 acts in a series with a lever
with an MA of 3, the mechanical advantage of the lever combination will be 2 • 3, or 6.
Remember that mechanical advantage does not have units; it is a number only!
2. Modern technology creates new uses for machines.
Sophisticated machines are often based on or contain several simple machines.
o Machines built from individual atoms and molecules of material are the result of
nanotechnology. Most Nano machines are still in the experimental stage.
Nanotechnology could also be used to develop materials that repel water and dirt
and make cleaning jobs easy. Nano scale biosensors could be used to detect
harmful substances in the environment. Another possible use for nanotechnology
is in military uniforms that can change color—the perfect camouflage.
In the future, nanotechnology may change the way almost everything is designed
and constructed. As with any new technology, it will be important to weigh both
the potential risks and benefits.
o Robots are machines that work automatically or by remote control. They do jobs
in places where it is difficult or dangerous for people to work.
Many companies use robots to manufacture goods quickly and efficiently. Robots
are widely used for jobs such as welding, painting, and assembling products.
Robots do some repetitive work better than humans, because robots do not get
tired or bored. Also, they do the task in exactly the same way each time. Robots
are very important to the automobile and computer industries.
J. C. Booth Middle School
Physical Science (8A)
Coach Dave Edinger